1. Field of the Invention
The present invention relates generally to catalysts. In particular, the present invention relates to methods and compositions concerning molybdenum disulfide (MoS2) and carbon-containing molybdenum disulfide (MoS2-xCx) catalysts with novel nanostructures that exhibit improved catalytic activity for hydrotreating reactions involving hydrodesulfurization, hydrodenitrogenation, and hydrogenation.
2. Description of Related Art
Hydrotreating processes are well known to the petroleum refining industry. These processes involve treating various hydrocarbon feeds with hydrogen in the presence of catalysts to lower the molecular weight of the hydrocarbons or to remove or to suitably alter the unwanted components. Hydrotreating may be applied to a variety of feedstock such as solvents, distillate feeds (light, middle, heavy), residual feeds, and fuels. In the treatment of catalytic cracking feedstock, the cracking quality of the feedstock is improved by the hydrogenation. For example, carbon yield may be reduced in order to increase gasoline yield.
When hydrotreating is used to remove unwanted compounds (e.g., sulfur, nitrogen, aromatics), unsaturated hydrocarbons are hydrogenated, and the saturated sulfur and nitrogen are removed. In the hydro-desulfurization of relatively heavy feedstock, emphasis is on the removal of sulfur from the feedstock, which is usually converted into lower molecular weight or lower boiling point components. In the hydrodesulfurization of heavier feedstock, or residues, the sulfur compounds are hydrogenated and cracked. Carbon-sulfur bonds are broken, and the sulfur for the most part is converted to hydrogen sulfide which can be removed as a gas from the process. Similarly, hydrodenitrogenation involves hydrogenating and cracking heavier feedstock or residues in order to remove nitrogen. Carbon-nitrogen bonds are broken, and the nitrogen is converted to ammonia and evolved from the process. In the hydrodenitrogenation of relatively heavy feedstock emphasis is on the removal of nitrogen from the feedstock, which is also converted to lower molecular weight or lower boiling point components.
The dwindling supplies of high grade petroleum feedstock necessitates the increased production and processing of transportation fuels from lower grade, heavy petroleum feedstock and synthetic liquid hydrocarbons derived from hydrocarbon-containing, or precursor hydrocarbon-containing solids. The refiners' feedstock sources as a result thereof continue to change, particularly as the worldwide supplies of petroleum diminish. The newer feedstock often contain higher amounts of nitrogen, sulfur, and other materials. Nonetheless, whatever the difficulties, it remains a necessity to effectively hydrotreating the new low quality feedstock often to a greater extent than was previously required due to more stringent regulations. In addition, these low-grade feeds with their high concentrations of sulfur, nitrogen, and aromatics cause activity suppression and an all too rapid deactivation of currently known catalysts. Coke formation is increased, which thus requires more cracking in order to achieve increased gas production.
Accordingly, considerably more upgrading is required to obtain usable products from these sources. Such upgrading generally necessitates hydrotreating the various hydrocarbon fractions, or whole crudes, and includes reactions such as hydrogenating to saturate olefins and aromatics, hydrodesulfurizing to remove sulfur compounds, hydrodenitrogenating to remove nitrogen, and conversion of high boiling compounds to lower boiling compounds.
Conventional hydrotreating catalysts include molybdenum disulfides (MoS2) promoted with nickel or cobalt, and they may be unsupported or supported (e.g., on alumina). The Co and Ni act as promoters for increasing hydro-treatment activity. In the following paragraphs, some conventional catalyst solutions of this general type are presented.
U.S. Pat. No. 4,243,554 to Naumann et al. (“Naumann”) teaches that cobalt and nickel promoted molybdenum disulfide catalysts with relatively high surface areas may be obtained through thermal decomposition of various ammonium thiomolybdate salts such as an ammonium salt of a molybdenum-sulfur cluster anion or an ammonium thiomolybdate salt having the formula (NH4)2 [MoOxS4-x], where x is 2. The decomposition of these thiosalts is carried out with sulfur-containing organic compounds in a hydrocarbon solution, pressurized with hydrogen at temperatures of about 300–800° C.
U.S. Pat. No. 4,508,847 to Chianelli et al. discloses a carbon-containing MoS2 catalyst. The carbon-containing molybdenum sulfide catalysts are obtained by contacting one or more catalyst precursors selected from (a) ammonium thiomolybdate or thiotungstate salts, (b) ammonium molybdate or tungstate salts, (c) substituted ammonium thiomolybdate or thiotungstate salts, (d) substituted ammonium molybdate or tungstate salts, and mixtures thereof, with sulfur, hydrogen and a hydrocarbon at a temperature broadly ranging from about 150 to 600° C. This produces catalysts that have the general formula MS2-zCz′ wherein 0.01≦z≦0.5 and 0.01≦z′≦3.0. These catalysts have surface areas of up to about 350–400 m2/gm. They can be promoted with one or more promoter metals such as cobalt. Such promotion produces catalysts having hydrorefining activity that is greater than that of prior cobalt molybdate on alumina hydrorefining catalysts.
U.S. Pat. No. 4,431,747 to Seiver et al. teaches a similar MoS2 catalyst. Seiver discloses supported carbon-containing molybdenum and tungsten sulfide hydrotreating catalysts, both promoted and unpromoted species, having high activity, selectivity, and stability especially in conducting hydrodesulfurization and hydrodenitrogenation reactions. In accordance therewith, a supported carbon-containing molybdenum sulfide and tungsten sulfide hydrotreating catalyst is formed by compositing a preselected quantity of a porous, refractory inorganic oxide with a complex salt characterized by the formula Bx[MOyS4-y], where B is an organo or hydrocarbyl substituted diammonium ion, an organo or hydrocarbyl substituted ammonium ion or quaternary ammonium ion, or an ionic form of a cyclic amine containing one or more basic N atoms, x is 1 where B is an organo or hydrocarbyl substituted diammonium ion, or 2 where B is an organo or hydrocarbyl substituted ammonium or quaternary ammonium ion or an ionic form of a cyclic amine containing one or more basic N atoms, M is molybdenum or tungsten, and y is 0, or a fraction or whole number ranging up to 3. A solution of the salt, or admixture of salts, is incorporated with a preselected quantity of a porous, refractory inorganic oxide support such as a particulate mass of the support. The salt-containing support is then dried to remove all or a portion of the solvent from the support, and the dried particulate salt-containing support is then heated in the presence of hydrogen, hydrocarbon, and sulfur or a sulfur-bearing compound to the decomposition temperature of the salt, or salts, to form the catalyst.
U.S. Pat. Nos. 4,528,089 and 4,650,563 also disclose carbon-containing molybdenum sulfide catalysts. The catalysts are formed by heating one or more precursor salts in the presence of sulfur and under oxygen-free conditions. The salts contain a thiometallate anion of Mo, W, or a mixture thereof and a cation that includes one or more promoter metals. The promoter metals are chelated by at least one neutral, nitrogen-containing polydentate ligand, with the promoter metal being Ni, Co, Zn, Cu or a mixture thereof. The precursor salts have the general formula, ML (MoyW1-yS4) where M is one or more divalent promoter metals such as Ni, Co, Zn, Cu or a mixture thereof. Y is any value ranging from 0 to 1, and L is one or more, neutral, nitrogen-containing ligands with at least one being a chelating polydentate ligand. Ideally, M is Co, Ni, or a mixture thereof, and the ligand, L, has a denticity of six and is either three bidentate or two tridentate chelating ligands. It is claimed that these catalysts have hydrotreating or hydrorefining activities substantially greater than those of catalysts derived from conventional hydrotreating catalyst precursors such as cobalt molybdate on alumina, even though their surface areas are not as high.
U.S. Pat. Nos. 4,581,125 and 4,514,517 disclose molybdenum disulfide catalysts that can be obtained by heating one or more carbon-containing, bis (tetrathiometallate) catalyst precursor salts selected from (NR4)2[M(WS4)2] or (NR4)x [M(MoS4)2] groups in a non-oxidizing atmosphere in the presence of sulfur and hydrogen at a temperature above about 150° C. for a time sufficient to form the catalyst. The (NR4) is a carbon-containing, substituted ammonium cation and R is either: (a) an alkyl group, aryl group, or mixture thereof, or (b) a mixture of (a) with hydrogen. Promoter metal, M, is covalently bound in the anion and is Ni, Co, or Fe. X is 2 if M is Ni, and x is 3 if M is Co or Fe. It is taught that the catalyst should ideally be formed in the presence of a hydrocarbon.
U.S. Pat. No. 4,839,326 discloses a catalyst formed by treating molybdenum sulfide- or tungsten sulfide-containing materials supported with an organometallic complex containing a transition metal promoter such as Co, Fe, and Ni. Similarly, U.S. Pat. No. 4,820,677 teaches a catalyst formed from an amorphous sulfide of iron and a metal selected from Mo, W, and mixtures thereof, along with a metal sulfide of at least one metal that has Co, Ni, Mn, Zn, Cu or a mixture thereof. The resulting catalyst is an amorphous sulfide of a mixture of iron with molybdenum and/or tungsten and, optionally, a mixture of the amorphous sulfide with a metal sulfide of one or more additional metals such as Ni, Co, Mn, Zn, and Cu.
U.S. Pat. No. 4,279,737 discloses chalcogenides that are superior catalysts for the treatment of hydrocarbons. They have the general formula, MXy where M is ruthenium, osmium, rhodium, or iridium, X is sulfur, selenium, tellurium, or a mixture thereof, and y is a number ranging from about 0.1 to about 3. The catalysts are prepared through a low temperature, nonaqueous precipitation technique.
Song et al., U.S. Pat. No. 6,156,693, teaches a method for preparing MoS2 catalysts by decomposing ammonium tetrathiomolybdate (ATTM) precursors dissolved in a solution of relatively high boiling point solvent (n-tridecane, boiling point=234° C.) and added H2O under H2 pressure at 350–400° C. The reference teaches that the MoS2 produced from ATTM and H2O at 350–400° C. has higher surface areas (286–335 m2/g) than those from ATTM without water, which have surface areas of 54–70 m2/g. Song teaches that the surface area of MoS2 prepared at 375° C. from ATTM and water is 342 m2/g, which is about three times that of MoS2 from ATTM without water (70 m2/g). It was also recognized that while water is effective for generating highly active catalyst, it actually impairs the catalytic conversion process itself. Therefore, the reference teaches that the water should be removed after ATTM decomposition to yield a more active MoS2 catalyst.
Finally, U.S. Pat. No. 6,299,760 discusses the production of unsupported molybdenum-containing catalysts; however, the method employed is different and the result catalyst is different.
In addition to these discoveries, several researchers have identified some general principles relating to the decomposition of ATM into molybdenum disulfide. For example, the decomposition of ammonium and amine thiosalts for creating molybdenum catalysts has been reported by Alonso et al., 1998a; Alonso et al., 1998b. These references teach that the decomposition of ATM (ammoniumthiomolybdate) at relatively low temperatures (e.g., 623 K) and high pressure (e.g., 3.1 MPa) hydrogen environment can produce a very disordered MoS2 structure with large surface areas and higher catalytic activity. Also, for non-mechanically pressed decomposition, the surface area and catalytic activity will increase with increase of bulkiness in the alkyl radical. In addition, it was confirmed that in situ decomposition results in more active catalysts than ex situ decomposition. Similarly, in Brito et al., (1995), it was reported that the overall thermal decomposition of ammonium thiomolybdate (ATM) to molybdenum disulfide in inert atmospheres generally occurs over a wide temperature range of 120–820° C. Initially, molybdenum trisulfide is formed between 120 and 260° C., and then MoS2 forms in the remaining range of between 300 to 820° C. While most of the reaction will occur in the range from 300 to 500° C., temperatures exceeding 800° C. are required to remove the remaining sulfer and achieve stoichiometric MoS2. However, hydrogen can be used to accelerate the decomposition of MoS3 to MoS2. In the presence of hydrogen, stoichiometric MoS2 can be formed at temperatures lower than 450° C.
Catalysts exist for the hydrotreating processes. However, a need exists for more efficient catalysts such as those provided by the present invention.
The processes and catalysts presented herein are very useful for hydrotreating reactions involving hydrosulfurization, hydrodenitrogenation and hydrogenation. These catalysts are useful in oil refining and production of petrochemical compounds.
The catalysts may be compacted and provided in a pellet form. This form may reduce the adsorption of water in the active catalyst.